Conventional audio communication systems transmit speech and audio signals in frames, meaning that the sending side first arranges the signal in short segments or frames of e.g. 20-40 ms which subsequently are encoded and transmitted as a logical unit in e.g. a transmission packet. The receiver decodes each of these units and reconstructs the corresponding signal frames, which in turn are finally output as continuous sequence of reconstructed signal samples. Prior to encoding there is usually an analog to digital (A/D) conversion step that converts the analog speech or audio signal from a microphone into a sequence of audio samples. Conversely, at the receiving end, there is typically a final D/A conversion step that converts the sequence of reconstructed digital signal samples into a time continuous analog signal for loudspeaker playback.
However, such transmission system for speech and audio signals may suffer from transmission errors, which could lead to a situation in which one or several of the transmitted frames are not available at the receiver for reconstruction. In that case, the decoder has to generate a substitution signal for each of the erased, i.e. unavailable frames. This is done in the so-called frame loss or error concealment unit of the receiver-side signal decoder. The purpose of the frame loss concealment is to make the frame loss as inaudible as possible and hence to mitigate the impact of the frame loss on the reconstructed signal quality as much as possible.
Conventional frame loss concealment methods may depend on the structure or architecture of the codec, e.g. by applying a form of repetition of previously received codec parameters. Such parameter repetition techniques are clearly dependent on the specific parameters of the used codec and hence not easily applicable for other codecs with a different structure. Current frame loss concealment methods may e.g. apply the concept of freezing and extrapolating parameters of a previously received frame in order to generate a substitution frame for the lost frame.
These state of the art frame loss concealment methods incorporate some burst loss handling schemes. In general, after a number of frame losses in a row the synthesized signal is attenuated until it is completely muted after long bursts of errors. In addition the coding parameters that are essentially repeated and extrapolated are modified such that the attenuation is accomplished and that spectral peaks are flattened out.
Current state-of-the-art frame loss concealment techniques typically apply the concept of freezing and extrapolating parameters of a previously received frame in order to generate a substitution frame for the lost frame. Many parametric speech codecs such as linear predictive codecs like AMR or AMR-WB typically freeze the earlier received parameters or use some extrapolation thereof and use the decoder with them. In essence, the principle is to have a given model for coding/decoding and to apply the same model with frozen or extrapolated parameters. The frame loss concealment techniques of the AMR and AMR-WB can be regarded as representative. They are specified in detail in the corresponding standards specifications.
Many codecs out of the class of audio codecs apply for coding frequency domain techniques. This means that after some frequency domain transform a coding model is applied on spectral parameters. The decoder reconstructs the signal spectrum from the received parameters and finally transforms the spectrum back to a time signal. Typically, the time signal is reconstructed frame by frame. Such frames are combined by overlap-add techniques to the final reconstructed signal. Even in that case of audio codecs, state-of-the-art error concealment typically applies the same or at least a similar decoding model for lost frames. The frequency domain parameters from a previously received frame are frozen or suitably extrapolated and then used in the frequency-to-time domain conversion. Examples for such techniques are provided with the 3GPP audio codecs according to 3GPP standards.